A square column is typically manufactured through cold forming by using a hot rolled steel sheet (hot rolled steel strip) or plate as the raw material. Examples of the cold forming employed in manufacturing a square column include press forming and roll forming. When a square column is to be manufactured through roll forming using a hot rolled steel sheet as a raw material, it is a prevailing practice to first form a hot rolled steel sheet into a round steel pipe and then cold-form the round steel pipe into a square column. This method of manufacturing a square column through roll forming has an advantage of high productivity compared to a method of manufacturing a square column through press forming. However, according to the method of manufacturing a square column through roll forming, large work strain is introduced in the pipe axis direction as the sheet is formed into a round form. Moreover, during the process of cold-forming the round form into a square form, flat portions of the square column are subjected to bend-back forming in a direction opposite the direction in which bending into the round form had been performed. Accordingly, a square column manufactured through roll forming has a problem in that the yield ratio in the pipe axis direction tends to be high and the ductility and toughness tend to be degraded due to the Bauschinger effect or the like.
To address this problem, for example, Japanese Unexamined Patent Application Publication No. 08-246095 describes a method of manufacturing a steel material for a low-yield-ratio, high-toughness square column, the method including hot-rolling a steel at a heating temperature of 1150° C. to 1250° C. and finishing temperature of 800° C. to 870° C. and performing coiling at 500° C. to 650° C., the steel containing, in terms of % by weight, at least one selected from C: 0.03 to 0.25%, Si: 0.10 to 0.50%, Mn: 0.30 to 2.00%, P: 0.020% or less, S: 0.020% or less, O: 50 ppm or less, H: 5 ppm or less, Al: 0.150% or less, Ti: 0.050% or less, V: 0.100% or less, Nb: 0.080% or less, Zr: 0.050% or less, and B: 0.0050% or less, and N so as to satisfy the relationship N≦(⅕){(½)Al+(1/1.5)Ti+(1/3.5)V+(1/6.5)Nb+(1/6.5)Zr+B}.
Japanese Unexamined Patent Application Publication No. 03-219015 describes a method of manufacturing a square pipe with low yield ratio and good low-temperature toughness, in which a low-carbon steel pipe is heated to a temperature of Ac3—250° C. to Ac3—20° C., quenched at a cooling rate of 15° C./s or more, cold-formed into a square pipe, and tempered at 200° C. to 600° C. According to Japanese Unexamined Patent Application Publication No. 03-219015, post-intercritical-anneal quenching, cold-forming, and tempering are sequentially performed to eliminate the effect of work hardening occurred during pipe forming and thus a square pipe with low yield ratio and high toughness can be manufactured.
Japanese Unexamined Patent Application Publication No. 2002-241897 does not explicitly describe a steel sheet for a square column. However, a steel sheet having high formability and low yield ratio is described therein. The steel sheet described in Japanese Unexamined Patent Application Publication No. 2002-241897 contains, on a % by mass basis, C: 0.0002 to 0.1%, Si: 0.003 to 2.0%, Mn: 0.003 to 3.0%, and Al: 0.002 to 2.0%, one or more groups selected from Group 1 including B: 0.0002 to 0.01%, Group 2 including a total of 0.005 to 1.0% of at least one selected from Ti, Nb, V, and Zr, Group 3 including a total of 0.005 to 3.0% of at least one selected from Cr, Mo, Cu, and Ni, and Group 4 including Ca: 0.005% or less and a rare earth element: 0.20% or less, and, as impurities, P: 0.0002 to 0.15%, S: 0.0002 to 0.05%, and N: 0.0005 to 0.015%, in which a mean crystal grain diameter of a ferrite phase is more than 1 μm but not more than 50 μm, the volume ratio of the ferrite phase is 70% or more, the aspect ratio of the ferrite phase is 5 or less, 70% of ferrite grain boundaries are high-angle grain boundaries, and the mean crystal grain diameter of a second phase, whose volume fraction among the rest of the phase is maximum, is 50 μm or less. This steel sheet has little variation in yield strength and yield ratio.
WO 2005/028693 A1 describes a hot rolled steel sheet for processing. The hot rolled steel sheet described in WO2005/028693 A1 has a composition of, on a % by weight basis, C: 0.01 to 0.2%, Si: 0.01 to 0.3%, Mn: 0.1 to 1.5%, Al: 0.001 to 0.1%, and P, S, and N adjusted to a particular value or less, and has a microstructure including a polygonal ferrite primary phase and a hard second phase, the volume fraction of the hard second phase being 3 to 20%, the hardness ratio (hard second phase hardness/polygonal ferrite hardness) being 1.5 to 6, and the grain diameter ratio (polygonal ferrite grain diameter/hard second phase grain diameter) being 1.5 or more. According to WO 2005/028693 A1, a hot rolled steel sheet that obtains a BH amount of 60 MPa or more can be manufactured by introducing strain through pressing and by performing bake hardening, and a press-formed part having a strength comparable to that achieved by a 540-640 MPa-grade steel sheet can be stably manufactured from a 370-490 MPa-grade hot rolled steel sheet.
Japanese Unexamined Patent Application Publication No. 2001-303168 describes a method of manufacturing a steel sheet having a good brittle crack property. According to Japanese Unexamined Patent Application Publication No. 2001-303168, a steel sheet having a microstructure constituted by a ferrite structure and a pearlite structure and having a composition that satisfies C: 0.03 to 0.2%, Si: 0.5% or less, Mn: 1.8% or less, Al: 0.01 to 0.1%, and N: 0.01% or less is obtained by hot-rolling, and that steel sheet is subjected to first cooling that includes cooling a region 5 to 15% in terms of thickness from a front surface of the steel sheet and a region 5 to 15% in terms of thickness from a back surface of the steel sheet at an average cooling rate of 4 to 15° C./s to a temperature of 450 to 650° C. or less. Then, the steel sheet is recuperated to a temperature not more than the Ar3 transformation temperature and subjected to second cooling at an average cooling rate of 1 to 10° C./s. As a result, the regions 5 to 15% in terms of thickness from the front surface and the back surface of the steel sheet come to contain fine ferrite grains with an equivalent circle mean diameter of 4 μm or less and an aspect ratio of 2 or less and the region 50 to 75% of the sheet thickness comes to contain fine ferrite grains with an equivalent circle mean diameter of 7 μm or less and an aspect ratio of 2 or less. Accordingly, a steel sheet having good COD properties, low-temperature toughness, and good brittle crack resistance can be obtained.
However, a steel material manufactured in Japanese Unexamined Patent Application Publication No. 08-246095 has a yield ratio of about 81 to 85% at the lowest and fails to achieve a low yield ratio of 80% or less. Moreover, the absorbed energy at 0° C. is sometimes less than 100 J. Thus, there is a problem in that high toughness cannot be stably achieved. According Japanese Unexamined Patent Application Publication No. 03-219015, two different types of heat treatment, namely, quenching after intercritical annealing and tempering, need to be performed and there is a problem in that the process is thus complicated, resulting in decreased productivity and increased manufacturing cost.
When a steel sheet described in Japanese Unexamined Patent Application Publication No. 2002-241897 is used as a raw material, formed into a round steel pipe, and cold-formed into a square column, the degree of cold working is high at the flat portions of the square column. Thus, there is a problem in that the square column may not always achieve sufficient toughness. When a steel sheet described in WO 2005/028693 A1 is used as a raw material, formed into a round steel pipe, and cold-formed into a square column, the degree of cold working is high at the flat portions of the obtained square column and thus there is a problem in that the yield strength and then the yield ratio are increased, and the toughness is decreased. Moreover, the hot rolled steel sheet described in WO 2005/028693 A1 is susceptible to strain aging and is thus not suitable as a raw material for manufacturing a square column by cold forming.
When a hot rolled steel sheet manufactured in Japanese Unexamined Patent Application Publication No. 2001-303168 is used and cold-formed into a square column, the yield strength of the square column obtained by cold forming increases and, as a result, the yield ratio increases, because the ferrite grains in this hot rolled steel sheet are fine. Accordingly, when a hot rolled steel sheet manufactured by the technology described in Japanese Unexamined Patent Application Publication No. 2001-303168 is used as a raw material, the resulting square column cannot achieve a low yield ratio of 80% or less needed for building structural members.
It could therefore be helpful to provide a hot rolled steel sheet suitable as a raw material for a square column for building structural members, the hot rolled steel sheet having strength of 215 MPa or more in terms of yield strength and 400 to 510 MPa in terms of tensile strength, a low yield ratio of 75% or less, and high toughness of 180 J or more in terms of absorbed energy in a Charpy impact test performed at a test temperature of 0° C. and preferably −30° C. We also provide a method of manufacturing the hot rolled steel sheet.